1. Field of the Disclosure
The present disclosure relates to a liquid crystal display device, and more particularly, to a power supplying unit having a linear low temperature compensating circuit that linearly compensates a gate high voltage according to a temperature and a liquid crystal display device including the power supplying unit.
2. Discussion of the Related Art
Recently, as information age progresses, demand for display device has increased in various forms. In addition, various flat panel displays (FPDs) having a thin profile, a light weight and a low power consumption such as a liquid crystal display (LCD), a plasma display panel (PDP) and an organic light emitting diode (OLED) have been researched.
Among various FPDs, a liquid crystal display (LCD) device is one of the most widely utilized FPDs. The LCD device includes two substrates having a pixel electrode and a common electrode, respectively, and a liquid crystal layer between the two substrates. In the LCD device, an alignment direction of liquid crystal molecules of the liquid crystal layer is determined according to an electric field generated by voltages applied to the pixel electrode and the common electrode and an image is displayed by controlling polarization of incident light according to the alignment direction.
In addition, since the LCD device has advantages such as high contrast ratio and superiority in displaying a moving image, the LCD device has been used for a monitor of a computer or a television as well as a display unit of a mobile terminal by substituting for a cathode ray tube (CRT).
The LCD device includes a gate driving unit supplying a gate signal to a display panel. Recently, a gate-in-panel (GIP) type LCD device where the gate driving unit is integrated in the display panel has been widely used. Since the gate driving unit of the GIP type LCD device includes an amorphous silicon thin film transistor (a-Si TFT), the GIP type LCD device is susceptible to a temperature as compared with a conventional LCD including a gate driving unit of a driving integrated circuit (D-IC) that includes a single crystalline silicon TFT. Specifically, as a temperature decreases, an ON-current of a pixel TFT decreases. Since a data signal is not sufficiently supplied to each pixel while the pixel TFT is turned on, display quality of the LCD device is deteriorated.
To prevent deterioration in display quality, an LCD device including a low temperature compensating circuit for supplying a boosted gate high voltage (VGH) that turns on the pixel TFT when an ambient temperature is lower than a reference temperature has been suggested.
FIG. 1 is a block diagram showing a power supplying unit for a liquid crystal display device according to the related art. In FIG. 1, a power supplying unit 10 includes a power integrated circuit (IC) 20, a charge pumping part 30 and a low temperature compensating part 40.
The power IC 20 generating a plurality of voltages for various units of a liquid crystal display (LCD) device includes a boosting part 22 generating a source voltage VDD and a level shifting part 24 sequentially supplying a gate high voltage VGH to a gate driving unit according to a control signal of a timing controlling unit (not shown).
The charge pumping part 30 boosts up the source voltage VDD and generates the gate high voltage VGH having different voltages according to a temperature.
The low temperature compensating part 40 controls the charge pumping part 30 so that the charge pumping part 30 can generate the gate high voltage VGH having different voltages according to an ambient temperature. For example, when the ambient temperature is equal to or higher than a reference temperature, the charge pumping part 30 may generate the gate high voltage VGH having a voltage twice as high as the source voltage VDD according to control of the low temperature compensating part 40. In addition, when the ambient temperature is lower than the reference temperature, the charge pumping part 30 may generate the gate high voltage VGH having a voltage three times as high as the source voltage VDD according to control of the low temperature compensating part 40.
Accordingly, the power IC 20 supplies a higher gate high voltage VGH in a lower ambient temperature so that deterioration in display quality can be prevented by compensating charge of each pixel by a data signal through a pixel TFT.
FIG. 2 is a view showing a gate high voltage outputted from a power supplying unit for a liquid crystal display device according to the related art. In FIG. 2, the power supplying unit 10 (of FIG. 1) outputs a gate high voltage VGH of a first voltage V1 when an ambient temperature is equal to or higher than a reference temperature Tref, and outputs the gate high voltage VGH of a second voltage V2 when the ambient temperature is lower than the reference temperature Tref. The first voltage V1 may be twice as high as a source voltage VDD (V1=2VDD) and the second voltage V2 may be three times as high as the source voltage VDD (V2=3VDD). As the gate high voltage VGH increases from the first voltage V1 to the second voltage V2, an ON-current of a pixel thin film transistor (TFT) increases. Since the ON-current of the pixel TFT in low temperature surroundings is compensated, each pixel is sufficiently charged by a data signal in a shorter time period and deterioration in display quality is prevented.
However, since the gate high voltage VGH generated by the related art power supplying unit 10 including the charge pumping part 30 (of FIG. 1) and the low temperature compensating part 40 (of FIG. 1) is restricted to one of the first and second voltages V1 and V2 that respectively are twice and three times as high as the source voltage VDD, there exists a problem such that an unnecessary voltage is supplied. The gate high voltage that is required for sufficiently charging each pixel with the data signal while the pixel TFT is turned on in low temperature surroundings increases according to decrease of the ambient temperature. However, since the gate high voltage VGH supplied from the power supplying unit 10 is restricted to one of the first and second voltages, the excessive gate high voltage is used in the LCD device and power consumption of the LCD device increases.
For example, in a linear section of FIG. 2 where the ambient temperature is lower than the reference temperature Tref, the required gate high voltage linearly increases under the second voltage V2 according to decrease of the ambient temperature and the power supplying unit 10 supplies the gate high voltage VGH of the second voltage V2. As a result, the power loss corresponding to difference between the supplied gate high voltage and the required gate high voltage is generated to cause increase in power consumption of the LCD device. In addition, since the low temperature compensating part 40 is separately formed outside the power IC 20, fabrication cost of the LCD device increases. Further, when the ambient temperature is steeply changed, the gate high voltage VGH cannot rapidly transition from the first voltage V1 to the second voltage V2. As a result, deterioration in display quality such as a block dim occurs.